I know that the Russian Federation retained the vast USSR nuclear arsenal, including the weapons in the non-Russia soviet bloc. I was zero years old when the USSR dissolved, and I am not a history buff, so my question may be naive. Nonetheless, why didn't Western powers attempt to seize and/or destroy the USSR nuclear arsenal?
My guess is that the U.S. did not want to break longstanding nuclear treaties, nor did it want to invade Russia. However, that's just my speculation.
You've pretty much answered your own question. Trying to seize the stockpiles or destroy the stockpiles would have involved invading/attacking Russia, which would have started a war. Which is not a good idea if the other guy has nuclear weapons.
For the past two decades, after one of the great diplomatic achievements in recent years, 10 percent of electricity consumed in the United States has come from nuclear material from upwards of 20,000 decommissioned Soviet era nuclear warheads.
After the Cold War concluded, U.S. government representatives persuaded the Russians to sell their surplus enriched uranium. Weapons-grade plutonium has over 9 percent Pu-239 and can be used, like reactor-grade plutonium, in fuel for electricity production.
Russia's nuclear industry badly needed the funding. The decaying Soviet nuclear arms complex had nearly a million workers not getting paid a living wage. There was a clear and present danger some of the nuclear material laying around unguarded would go to the black market.
Beginning in 1993, the Russians turned about 500 tons of bomb-grade uranium into nuclear fuel. The U.S. bought it to sell to commercial power plants. The Russians made around $17 billion and American power plants got the uranium at a good price.
The collapse of the USSR left Russia a destabilized, uncertain, poor country — but one armed to the teeth with both nuclear and conventional weaponry. The hopes of the USA was that Russia would become a functional, friendly democracy. If the US had launched any kind of attack there would have been a terrible retribution. And the Soviet arsenal at that point, like the American arsenal, was far too large to imagine seizing — it numbered in the many tens of thousands of warheads dispersed across the nation's land mass and under the oceans in submarines.
What did happen was that the USA and Russian Federation did embark on a series of arms cut-backs, and the USA helped finance Russian nuclear security infrastructure, and the USA did buy considerable excess fissile material from Russia and used it to fuel US civilian nuclear reactors. All of these efforts were an attempt to scale back the nuclear danger in a sane, practical fashion.